Mercury has become recognized as a non-benign environmental pollutant. Materials and wastes can be contaminated by mercury from a number of sources including chemical spills and decommissioned chlor-alkali plants.
A significant but decreasing percentage of the world's chlorine and caustic soda comes from mercury amalgam cell chlor-alkali plants. These plants produce contaminated wastewater and sediments which must be treated to remove the contaminants before disposal to the environment. A common practice is to treat the wastewater with a combination of acidification and sulfidation to remove soluble mercury as mercuric sulfide. The wastewater is clarified prior to disposal while the solids containing mercuric and mercurous compounds as well as metallic mercury have in the past been disposed of in a hazardous landfill. The mercury content of the solids can vary significantly but is typically 1-6% wt. total mercury.
Recently, the United States Environmental Protection Agency (EPA) has enacted a number of new rules which regulate the disposal of industrial waste streams. The rule making procedure has identified special hazardous waste streams such as the chlor-alkali plant mercury contaminated wastewater treatment filter cake designated as K-106. The K-106 material is classified as a high mercury sub-category waste and will in the future be banned from land fill disposal.
The EPA has designated retort/roasting as the standard treatment technology (Best Demonstrated Available Technology BDAT) for treating K-106 material. The technology is well established, having been used extensively for the recovery of mercury from cinnabar ores and for the purification of contaminated mercury (triple distillation). There are, however, numerous problems associated with retorting of low concentration K-106 filter cakes. The most significant are:
1. The condenser recovery of mercury vapour from the low concentration retorter off-gas is poor. PA0 2. The condensed mercury is contaminated with sulfur and carbonaceous material thereby making further refining to triple distilled quality difficult. PA0 3. The high temperature chloride containing off-gas is highly corrosive. PA0 4. The sulfur must be recovered from the off-gas. PA0 5 The retort facility is expensive in terms of capital and operating cost.
U.S. Pat. No. 3,639,118, O'Grady, discloses a method for purifying mercury contaminated with metallic components (notably iron and calcium). Specifically, mercury materials having impurities therein are contacted with "nascent" chlorine in an aqueous phase capable of generating nascent chlorine so that the metal impurities are solubilized into the aqueous phase. Thereafter, the aqueous phase is removed, leaving behind relatively pure mercury. In a preferred embodiment, the O'Grady process involves the use of ". . . a mineral acid and an oxychloride salt e.g., sodium hypochlorite . . . " (col. 1, lines 39-40) which are reacted in the presence of the mercury.
After the mercury is treated in the O'Grady process using the above-identified chemical system, it is thereafter separated and washed with water. In the alternative, washing may be accomplished simultaneously with separation as stated in column 1, lines 59-60.
In essence, O'Grady teaches a technique for purifying metallic mercury that is contaminated with other metallic components. O'Grady's objective is to remove the impurities in the mercury, thereby leaving behind the pure mercury. This is different from a process where the objective is to remove both mercury and mercury compounds from a solids matrix to yield an uncontaminated matrix which is free of mercury and mercury compounds.
U.S. Pat. No. 5,013,358, granted May 7, 1991, Ball et al., discloses and claims a method for the recovery of mercury from mercury-containing material. In the process, insoluble mercury or mercury salts in mercury-containing material are converted into a soluble form by controlled chlorination. The soluble forms of mercury in the chlorination solution are reduced with iron, preferably iron powder, to yield elemental mercury. After separation from the reduced solution, the solids from the reduction containing entrained mercury, are subjected to a separation procedure for the separation and quantitative recovery of substantially pure mercury. Separation by elutriation through a body of mercury is preferred. Prior to separation, the reduction solids may be kneaded for coalescence of fine mercury particles, followed by slurrying of the kneaded material. Any selenium in the reduced solution may be recovered in a reduction with a suitable reductant, preferably by adding strong sulfuric acid in the presence of the ferrous chloride formed in the preceding reduction, and excess sulfur dioxide. The process is carried out at ambient conditions. The amount of liquid in the process is controlled. Substantially no mercury is discharged from the process in residues, or residual liquid.
The Ball et al. process utilizes chlorination in order to convert the insoluble mercury salts or mercury into soluble forms. Ball et al. do not disclose or teach concentrating mercury containing muds (K106 muds), treating the concentrated muds with acid and sodium hypochlorite in order to leach the muds, and then subsequently concentrating the mercury containing materials further before passing the materials through a second leach of acid and sodium hypochlorite. Ball et al. also do not teach a countercurrent mud treatment process using overflow from the first and second leaches.